1. Field of the Invention
The present invention relates to an X-ray image processing apparatus for creating the image of an X-ray characteristic distribution in a subject by using ordinary rays, such as an X-ray computer tomography (CT) apparatus for photographing an image by using X-rays or the like, and an X-ray image processing method.
2. Description of the Related Art
Hitherto, known X-ray CT apparatuses for exposing a subject to X-rays, detecting an X-ray that passes through the subject or that is scattered by the subject using an X-ray detector, and picking up the fluoroscopic image, tomographic image, and three-dimensional image of the subject based on the X-ray detection output (the number of photons of the X-ray) have been used.
However, the subject must be fixed for collecting data obtained during scanning performed by the X-ray CT apparatus. Further, the body motion and/or the internal-organ motion during the scanning generates an artifact in a reconstructed image. Subsequently, the diagnostic efficiency reduces.
Japanese Patent Laid-Open No. 2000-217810 and Japanese Patent Laid-Open No. 2002-355241 disclose cone-beam CT apparatuses, as known systems for solving the above-described problems. The cone-beam CT apparatuses obtain a transmission X-ray image of the subject by emitting an X-ray to the subject from circumference, and create and display an X-ray tomography image and/or a three-dimensional X-ray image of the subject based on the transmission X-ray image. According to the above-described disclosures, the transmission X-ray images are divided into different groups, and a transmission X-ray image corresponding to each group is reconstructed, so that a plurality of X-ray distribution images is obtained. Then, the sharpness of each of the X-ray distribution images is calculated and an X-ray tomography image and/or a three-dimension X-ray image of the subject is created, based on the transmission X-ray image of a group showing the highest sharpness, and displayed. More specifically, various half-scan data sets with shifted start angle are generated from full-scan data. Then, reconstructed images created by using the various half-scan data sets are compared to one another and a suitable image is selected.
An imaging system may be used for solving the above-described problems. The imaging system scans the heart of a patient, obtains a plurality of projection views, and determines a differential projection by the projection views. A load function for minimizing the motion artifact is used for the differential projection, and an inequality index used for determining the image reconstruction position is determined by the differential projection. According to this method, an electrical signal is not measured but the dynamic motion of the heart is directly measured. Further, the most suitable position for minimizing an image artifact is selected by using the projection data. More specifically, by using the load function for the differential projection, as disclosed in Japanese Patent Laid-Open No. 2002-355241, the heart-pulsation phase can be detected. Subsequently, a reconstructed image is obtained by using half-scan data from which images showing major heart-pulsation motion are moved.
The cone-beam CT apparatus have been developed, as X-ray CT apparatuses. In the case of the known X-ray CT apparatuses, an X-ray beam is sliced in a vertical direction Z and referred to as a fan beam. However, in the case of the cone-beam CT (CBCT), an X-ray beam extending in the direction Z is used. Therefore, the X-ray beam is referred to as a cone beam.
In the CBCT apparatus that is different from known CT apparatuses wherein a small number of slices are collected at the same time, several hundred slices of data are collected at the same time. That is to say, the projection data includes part inducing artifacts and another part inducing no artifacts. In general, the quality of an image reconstructed by using the full-scan data is better than that of an image reconstructed by using the half-scan data. Therefore, even though an artifact occurs in part of the projection data, the entire projection data should not be reconstructed by half scanning from the viewpoint of making effective use of the projection data.